The present invention relates to a control system for a belt-type continuously variable transmission, particularly to achievement of appropriate control of surplus lateral pressure to a belt (clamp pressure) in the belt-type continuously variable transmission.
A belt-type continuously variable transmission has been known and used which includes a driving pulley in which a pulley width is adjustable, a driven pulley in which a pulley width is adjustable, and a belt member wound around between the driving pulley and the driven pulley. The transmission has a driving-side hydraulic actuator for controlling the pulley width (axial thrust control) of the driving pulley and a driven-side hydraulic actuator for controlling the pulley width (axial thrust control) of the driven pulley. By providing hydraulic pressure to both the hydraulic actuators, axial thrusts of both the pulleys are controlled to adjust settings of the pulley widths, thereby variably setting a gear change ratio in a nonstep manner.
In a control system for such a kind of belt-type continuously variable transmission, to prevent belt slip, the hydraulic pressure (axial thrust) acting on the driven pulley is controlled to apply a minimum required pulley axial thrust (belt clamping force). The balance between the pulley axial thrusts to adjust the change gear ratio is set by controlling the hydraulic pressure (axial thrust) acting on the driving pulley. In such a case, the axial thrust of the driven pulley is determined with a belt-transmitted torque (torque transmitted between the pulleys) and the gear change ratio. A pulley axial thrust ratio between the driving pulley and the driven pulley is obtained from a target gear change ratio and a transmitted torque ratio. A pulley axial thrust deviation is obtained from dynamic speed change characteristics and a feedback element of the gear change ratio. Consequently, the driving-side axial thrust (hydraulic pressure) is set to the value composed of the sum of the product of the driving-side pulley axial thrust and the pulley axial thrust ratio and the pulley axial thrust deviation.
Japanese Patent Application Publication No. 2000-18347 (Patent Document 1) discloses that a target speed change can be achieved while preventing a belt slip by use of the minimum required pulley axial thrust even in such a speed change that the axial thrust of the driving pulley largely decreases.
Japanese Patent Application Publication No. Hei 5-79550 (Patent Document 2) discloses that control is performed in response to pulsation of hydraulic pressure.
In a belt-type continuously variable transmission (CVT), it is required to output hydraulic pressure not lower than the minimum required pressure as a lateral pressure to the belt (clamp pressure) to prevent belt slip. Accordingly, it was required to set a surplus pressure in consideration of factors lowering hydraulic pressure such as pulsation, environmental change, aging degradation, product variability (variability among individual CVT products), and the like. Although a technology has been developed such that a hydraulic pressure sensor is used to detect decrease in hydraulic pressure due to environmental change or aging degradation to regularly keep a surplus pressure low, a certain amount of surplus pressure must be secured to cope with instantaneous decrease in hydraulic pressure or occurrence of pulsation. However, securing a surplus pressure may result in decreased fuel efficiency and further in decreased durability of the belt; therefore it is desired to improve such a situation.